Zeolite Y, a member of the Faujasite family, is widely used in many catalytic processes such as fluid catalytic cracking (FCC), hydrocracking, aromatics alkylation, and aromatics transalkylation. A particular type of zeolite Y is known as ultra-stable Y zeolite (USY). Typical USY has a crystal morphology of non-aggregated and submicrosized crystals and may contain intra-crystal mesopores after post-treatment involving high temperature steaming. The individual submicrosized crystals may have crystal defects which produce variously oriented crystal grains within an individual crystal particle. The Kuvettu et al U.S. Pat. No. 6,284,218 states that such defects include stacking faults and screw defects.
When heavy molecules are processed by a catalyst, such as catalytic cracking of heavy gas oil, transport properties (both intra-particle and inter-particle) of the catalyst are important in order to operate outside of the diffusion limited regime which often leads to coking.
The article by K. Rajagopalan, A. W. Peters, G. C. Edwards, “Influence of Zeolite Particle Size on Selectivity During Fluid Catalytic Cracking”, Applied Catalysis, 1986, 23, 69-80, reports that smaller particle size NaY zeolite fluid catalytic cracking (FCC) catalysts exhibited improved activity and selectivity to intermediate cracked products, such as gasoline and light cycle oil. Selectivity differences were said to be explained by considering the effect of diffusion resistance on the rate constants for cracking of gas oil and gasoline.
The Absil et al U.S. Pat. No. 5,620,590 reports that small crystal zeolite Y of less than 1 micron shows activity benefit in hydrocracking compared to larger crystals. However, small crystal zeolites often present problems in manufacturing (e.g., difficulties in filtration and formulation) due to their small particle sizes and low bulk density. Therefore, it is desirable to have zeolites that possess the performance advantages of small particles, while still maintaining the easy processibility of large particles. Thus, an ideal morphology of zeolites is one that consists of large secondary particles (often greater than 1 micron) formed by agglomeration of smaller primary particles (often less than or even much less than 1 micron). Furthermore, to improve mass transportation rates, there is a need to make zeolite crystals with small size or aggregated crystals containing inter-crystal mesopores for reducing diffusion limitations.
Conventional zeolite Y tends to have a crystal or primary particle size of much greater than 0.1 microns, even greater than 1 micron. Examples of disclosures which describe such conventional forms of zeolite Y include, the Robsin U.S. Pat. No. 3,343,913, the Young U.S. Pat. No. 3,690,823, and the McDaniel et al U.S. Pat. No. 3,808,326.
Small crystal size zeolite Y may be prepared by methods disclosed in the Maher et al U.S. Pat. No. 3,516,786 and the Young U.S. Pat. No. 3,864,282.
Zeolite X, zeolite Y and natural faujasite have identical structure types and differ only in the ratio of silica to alumina in the final crystal structure. For example, zeolite X is generally referred to as having a silica to alumina molar ratio of 2 to 3, whereas zeolite Y is generally referred to as having a silica to alumina molar ratio of 3 to 7.
The Funkakoshi et al U.S. Pat. Nos. 5,993,773 and 6,306,363 describe various forms of low-silica faujasite zeolite, referred to as LSX, having silica to alumina molar ratio of 1.9 to 2.1. These patents include SEM photographs showing particle size and morphology of LSX zeolites.
In the Funkakoshi et al U.S. Pat. No. 6,306,363, it is stated that, when zeolites are observed by Scanning Electron Microscope (SEM), they may be visible either (1) in the form of non-aggregated primary particles only, which are the smallest units of zeolite particles, or (2) in the form of secondary particles which are formed by agglomeration of a plurality of primary particles. Usually, primary particles of zeolites have their shapes predetermined depending upon the type of zeolite. For example, A-type zeolite tends to have a cubic shape, and faujasite-type zeolite tends to have an octahedral shape or a polyhedral shape having a generally spherical shape with some angularity developed as shown in FIG. 3 of the Funkakoshi et al U.S. Pat. No. 6,306,363. However, it is possible for faujasitie-type zeolites to have other shapes, such as elongated shapes (e.g., flattened octahedral shapes, hexagonal slabs or rod-like shapes).
Usually, particle sizes of these particles have a distribution about a certain value at its center. A method for obtaining an average particle size from particles having a distribution, is described in detail, for example, at pages 1 to 31 in “Powder Engineering Theory”, edited by Shigeo Miwa, published in 1981 by Nikkan Kogyo Shinbun K. K. The primary particle may be described as a number average particle size of the diameters when the primary particles of the faujasite-type zeolite observed by a SEM are approximated to spheres which are called “projected area diameter” or “Heywood diameter”.
LSX in the Funkakoshi et al U.S. Pat. No. 6,306,363 is described as being of high purity and characterized in its fine primary particle size. LSX in the Funkakoshi et al U.S. Pat. No. 6,306,363 has a primary particle size of at least 0.05 μm and less than 1 μm, which is said to be a fine primary particle size in comparison with previously known forms of LSX having a primary particle size of from 3 to 5 μm, and even small forms of LSX having a primary particle size of at least 1 μm. In the Funkakoshi et al U.S. Pat. No. 6,306,363, it is stated that, when fine LSX of high purity is used, for example, as an adsorbent of various substances, diffusion into the interior will be facilitated, and improvement in various dynamic properties can be expected.
The LSX described in the Funkakoshi et al U.S. Pat. No. 5,993,773 is said to be characterized not only by high purity, but also a peculiar primary particle size distribution. The LSX of the Funkakoshi et al U.S. Pat. No. 5,993,773 comprises particles having a smaller primary particle size and particles having a larger primary particle size, wherein the primary particle size of the smaller particles is from 1 μm to 8 μm, the primary particle size of the larger particles is from 5 μm to 15 μm, and the particles having a smaller primary particle size accounts for 90% or more of the total particle number. The right hand portion of FIG. 2 of the Funkakoshi et al U.S. Pat. No. 5,993,773 illustrates a large single crystal or primary particle having a spherical polyhedral shape with angularity or edges developed.